Jinja2 syntax highlighting + description field + preview toggle

JinjaEditor:
- Custom StreamLanguage parser for Jinja2 syntax highlighting:
  {{ variables }} in blue, {% statements %} in purple, {# comments #} in gray
- Replaced HTML mode (didn't understand Jinja2 syntax)
- Proper monospace font (Consolas/Monaco)

TemplateConfig:
- Added `description` field to model + seed defaults with descriptions
- Description shown on template cards instead of raw template text
- Description input in create/edit form

Preview:
- Toggle behavior: clicking Preview again hides the preview
- Per-slot preview uses preview-raw API (renders current editor content)

i18n: added common.description, templateConfig.descriptionPlaceholder

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/lib/i18n/en.json

@@ -237,6 +237,7 @@
 		"newConfig": "New Config",
 		"name": "Name",
 		"namePlaceholder": "Default EN",
+		"descriptionPlaceholder": "e.g. English templates for family notifications",
 		"noConfigs": "No template configs yet.",
 		"eventMessages": "Event Messages",
 		"assetsAdded": "Assets added",
@@ -296,6 +297,7 @@
 		"cancel": "Cancel",
 		"delete": "Delete",
 		"edit": "Edit",
+		"description": "Description",
 		"close": "Close",
 		"confirm": "Confirm",
 		"error": "Error",